1. Field of the invention
The present invention relates, in general, to a solid electrolyte for sodium-sulfur secondary cell and, more particularly, to a solid electrolyte of beta alumina and/or .beta." alumina useful for a sodium-sulfur secondary cell, a sensor or a thermal pumping apparatus. Also, the present invention is concerned with a process for the preparation of solid electrolyte.
2. Description of the Prior Art
Since 1970, research and development efforts have been directed to sodium-sulfur secondary cells because they are superior to existing lead storage batteries in energy density and power density to weight in the order of two-fold. Sodium-sulfur secondary cells may be useful for drive devices utilizing electricity, such as electrical mobiles and the like, and may be adapted to electrical cells for storing power.
Usually, in sodium-sulfur secondary cells, a sintered body of beta alumina and/or .beta." alumina, which selectively penetrates only sodium ions at 350.degree. C. without electron conductivity, is employed as a solid electrolyte. A sodium-sulfur secondary cell is manufactured by contacting metallic sodium with one surface of the electrolyte of beta alumina and/or .beta." alumina with the other surface coming into contact with molten sulfur. Upon operating the sodium-sulfur secondary cell, the metallic sodium is ionized to allow current to flow through a current conduction body of metal, and then, the sodium ions move through the electrolyte into a place of molten sulfur and react with the molten sulfur, to produce compounds of formula Na.sub.2 S.sub.I wherein x is between 2.7 and 5.0. The current obtained through such reaction mechanism is utilized to drive the electrical apparatuses.
Sodium-sulfur secondary cells, on the other hand, can be recharged. For example, if current is forced to flow in the reverse direction, the sodium polysulfur compounds are decomposed into molten sulfur and sodium ions which then move through the electrolyte into their own original place and are reduced into metallic sodium thereat.
In addition, sodium-sulfur secondary cells should be discharged still molten sulfur is reacted with the sodium ions to produce up to Na.sub.2 S.sub.3. Over-discharge of sodium-sulfur secondary cells produces Na.sub.2 S.sub.2 and/or Na.sub.2 S therein, which compounds are precipitated at a temperature of 300.degree. to 350.degree. C., incapacitating selective separation of the sodium ions upon recharging. The sodium-sulfur secondary cells, therefore, come to be largely lowered in capability. This phenomenon, called "deep discharge", is a cause of reducing the life of the sodium-sulfur secondary cell.
In addition, it is required that both the charging rate and the discharging in sodium-sulfur secondary cell be not more than the critical current density of the beta alumina and/or .beta." alumina. Herein, the critical current density means the largest amount of current per unit area of electrolyte at which the electrolyte is not broken down in a variable of an electrolytic process.
Since most, e.g. about 70% of the total inner resistance generated in a secondary cell using beta alumina and/or .beta." alumina, such as sodium-sulfur secondary cell, is attributed to the solid electrolyte, it is important to reduce the resistance caused by the solid electrolyte. In addition, since mechanical strength of the solid electrolyte largely determines the life of sodium-sulfur secondary cell, it is preferred to prepare a sintered body of the solid electrolyte with higher mechanical strength.
Generally, .beta." alumina is superior to beta alumina in both Na ion conductivity and mechanical strength. So, rather than beta alumina, there is applied .beta." alumina, which can be prepared by adding Li.sub.2 O and MgO to a beta alumina composition, to the sodium-sulfur secondary cell as a solid electrolyte for use in insulating metallic sodium as a cathode active material and molten sulfur as an anode active material.
For the solid electrolyte of sodium-sulfur secondary cell, the beta alumina and/or .beta." alumina is necessary to be sintered. At 350.degree. C., the sintered body of beta alumina and/or .beta." alumina preferably has an ion conductivity of about 3.0 .OMEGA..multidot.cm and a mechanical strength of about 170 MPa. However, the sintered body of beta alumina and/or .beta." alumina has a disadvantage in that its optimum sintering temperature range is very narrow. When mass production in a batch type manner, the sintered body may be prepared at a variety of temperature because a large scale furnace, a sintering facility, has different temperatures according to its inner places. Owing to this, the prepared sintered body comes to have not uniform but variable properties.
Many an effort has been made to overcome the above problems. For example, after measuring practical temperature distribution in the furnace of which the inner temperature is fixed by a temperature controller, a composition most suitable to the temperature distribution is sintered therein. In this case, one sintering process may be enough for the sintered body of beta alumina and/or .beta." alumina with the above preferred properties, allowing high production yield.
In addition, research and development efforts have been directed to additives which give the electrolyte better properties. For example, ZrO.sub.2 is described in Japanese Patent Laid-Open Publication No. Sho. 59-141459 while TiO.sub.2 in Japanese Patent Laid-Open No. Heisei 3-279258. It is written in European Patent No. 495652 that addition of SnO.sub.2 mitigates the affects caused by the variable sintering temperature. In addition to those quadrivalent positive ion oxides, quinquevalent positive ion oxides, such as Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5, are disclosed in European Patent No. 471523.
However, the additives suggested in the supra patents cause some problems in a final sintered body. For example, ZrO.sub.2 wants to increase the sintering temperature according to its amount. In case of TiO.sub.2, Ti ions are unstable in atomic valence and are highly apt to be reduced, so that ion conductivity of the electrolyte is deteriorated. Further, it is difficult to control the properties of the sintered body of beta alumina and/or .beta." alumina with SnO.sub.2 because of its strong volatilization at the sintering temperature. The additive Ta.sub.2 O.sub.5, quinquevalent oxide, is very expensive relative to that used in the present invention.